Advancement in integrated circuit technology has lead to vast improvements in the speed of integrated circuits, i.e. the time in which the output of a circuit reacts in response to a new input. Increasing integrated circuit speed has resulted in faster rise and fall times of the output voltages. Similarly, the fast rise and fall times of the output voltages have resulted in abrupt transitions of output current.
While faster speeds are very desirable, the abrupt transition of output currents has created serious problems. The package which holds an integrated circuit device has metallic leads which allow interconnection of the device on a circuit board. Each lead has a small inductance associated with it. The leads are connected to the integrated circuit using bonding wire, which also has an inductance associated with it. Voltage is related to inductance and the time rate of change of current by the equation E=L.multidot.dI/dT, where L is the measure of inductance, and dI/dT is the change in current with respect to time. The abrupt transition of output currents creates a large change of current at the ground and power supply leads and in the bonding wire, resulting in ground and power supply voltage spikes. These voltage spikes affect the output voltages of the device, and cause output ringing, ground bounce, and false signals.
Since the power supply and ground nodes are used as voltage references throughout an integrated circuit, variations caused by inductive voltages on the power supply and ground nodes affect signals within the integrated circuit. For instance, if the voltage level of the ground node rises, the input signal will appear to decrease. Thus, if a slowly rising input signal produces an output voltage transition as the input signal rises above the voltage threshold, the resulting inductive voltage on the ground node may result in an effective decrease in the input signal voltage. This behavior can cause false signals and oscillations within the integrated circuit.
Previous circuits have used hysteresis to counter the voltage variations of the input signal due to inductive voltages. In these circuits, the input signal has a voltage range over which it can vary without being recognized as a signal of the opposite polarity. However, with high-speed integrated circuits, the inductive voltage on the power supply and ground nodes can easily exceed a reasonable voltage range.
From the foregoing, it may be seen that a need has arisen for a technique which prevents propagation of false input signals caused by inductive voltage present at the power supply and ground nodes.